Passing water to be treated through an ion-exchange resin to deionize the water has been known as a method for producing deionized water (hereinafter also referred to as “desalted water”). In this method, when the ion-exchange resin is saturated with ions, the ion-exchange resin needs to be regenerated by chemicals. In order to eliminate such a disadvantageous point in a treatment operation, a deionized water producing method, which uses an electric deionization that does not require the regeneration by the chemicals, has been established and put into practice.
In an electric device for producing deionized water that performs such a desalting treatment, for example, as shown in FIG. 1, basically, a chamber defined by a cation exchange membrane 107 and an anion exchange membrane 108 is filled with an ion exchanger, whereby desalting chamber 101 is constructed. Concentration chambers 102a, 102b are provided on both sides of desalting chamber 101. Desalting chamber 101 and concentration chambers 102a, 102b are interposed between two electrode chambers 103a, 103b, that is, between an anode chamber having an anode and a cathode chamber having a cathode, whereby main body part 104 is constructed. Water to be treated 105 is passed through a layer of the ion exchanger filled in desalting chamber 101. Voltage is placed between the anode and the cathode, thereby applying a direct current in a direction orthogonal to the flow of water to be treated 105 via both ion exchange membranes. In this way, impurity ions in water to be treated 105 are electrically removed into concentrated water flowing in concentration chambers 102a, 102b arranged outside both ion-exchange membranes, whereby deionized water is produced as treated water 106.
The type of main body part 104 of the electric device for producing deionized water includes multilayered plate type structure, a spiral type, and a concentric type.
In the case of an electric device for producing deionized water of multilayered plate type structure, a plurality of desalting chambers 101 each of which has an ion exchanger interposed between cation exchange membrane 107 and anion exchange membrane 108 are adjacently arranged via concentration chambers 102a, 102b into which ions are removed and have cathode chamber 103a and anode chamber 103b arranged on both ends thereof. FIG. 1 shows multilayered plate type structure having one desalting chamber 101 as an example.
Such an electric device for producing deionized water of multilayered palte type structure has an advantage in which the value of current applied to each chamber is uniform. However, the desalting chamber, the concentration chambers, the electrode chambers are arranged adjacently and are pressed to each other by fastening bolts or the like, so when the fastening force is weak, gaps are produced between the chambers to thereby raise the possibility of the occurrence of water leakage when water is passed through the chambers. Generally, in order to prevent water leakage, in the electric device for producing deionized water of multilayered plate type structure, sturdy fixing plates 109 are arranged on the outside of both ends in a direction in which the respective chambers of main body part 104 are arranged and fixing plates 109 are coupled to each other with bolts 111. Main body part 104 is fastened strongly by means of bolts 111.
Further, gaskets 110 are usually interposed between the respective chambers of desalting chamber 101, concentration chambers 102a, 102b, and electrode chambers 103a, 103b. Gaskets 110 seal gaps in contact portions of the respective chambers and hence prevent water leakage. Generally, a rubber sheet or a rubber ring is used as the gasket 110.
In a case where water to be treated at high temperature (40° C. or more) is passed through the electric device for producing deionized water of multilayered plate type structure described above, the main body part is thermally expanded by an increase in temperature above ambient temperature. However, in a case where the fixing plates and bolts are made of metal and the respective chambers are made of plastic, since the plastic is generally higher in the coefficient of thermal expansion than the metal, the main body part sandwiched by the fixing plates cannot be expanded in the direction in which the respective chambers are arranged but is expanded and deformed in a direction perpendicular to the direction in which the respective chambers are arranged. When the temperature returns to ambient temperature, the respective chambers contract but the deformed portions cannot perfectly return to their original shape and the respective chambers contract in the direction in which the respective chambers are arranged. As a result, gaps are produced between the respective chambers which results in water leakage.
In addition, since the composition of material of the main body part is not uniform, there is also the possibility that gaps will be produced even when the respective chambers are deformed by thermal expansion.
These conditions are likely to be caused, for example, in a desalting device for treating hot water. In particular, water leakage is not permitted to occur in a nuclear power plant and hence use of the current device is not allowed.